Iterative l-Tryptophan Methylation in Psilocybe Evolved by Subdomain Duplication

Area of Science:

• Biochemistry
• Mycology
• Evolutionary Biology

Background:

• Psilocybe mushrooms contain psilocybin, a psychotropic alkaloid derived from l-tryptophan.
• Psilocybin biosynthesis involves the enzyme PsiM, which methylates norbaeocystin.
• Psilocybe serbica exhibits unusual l-tryptophan methylation, inconsistent with PsiM's known specificity.

Purpose of the Study:

• To identify and characterize the enzyme responsible for the non-psilocybin l-tryptophan methylation in Psilocybe serbica.
• To elucidate the biochemical pathway and evolutionary origins of this novel methyltransferase.
• To propose a new mechanism for the evolution of secondary metabolism in fungi.

Main Methods:

• Enzyme identification and functional characterization.
• In vitro enzymatic assays using l-tryptophan and tryptamine as substrates.
• Comparative genomics and phylogenetic analysis to trace gene origins.

Main Results:

• A novel methyltransferase, TrpM, was identified and functionally characterized.
• TrpM exhibits mono- and dimethylation activity on l-tryptophan but not tryptamine.
• TrpM is evolutionarily linked to a duplicated ergothioneine biosynthesis gene (egtDB), suggesting a novel evolvability mechanism.

Conclusions:

• A second, independent l-tryptophan metabolic pathway exists in Psilocybe, not involved in psilocybin production.
• TrpM's origin from a conserved gene duplication provides a new model for secondary metabolism evolution.
• This mechanism highlights the retention of weakly selected genes through conserved pathways, enabling sporadic re-evolution.